Abstract

Background: Microsatellite instability (MSI) occurs in 10-20% of colorectal cancer (CRC), and has been attributed to both MLH1 promoter hypermethylation and germline mutation in the mismatch repair (MMR) genes. Inactivation of MLH1 due to promoter hypermethylation strongly suggests a sporadic origin, providing exclusion criteria for Lynch syndrome.

The aim of the study is to show the need to perform MLH1 methylation analysis in all MSI-H tumors before sequencing of MLH1 gene in Estonian clinical practice.

Methods: The study group included 50 tumors DNA from colorectal cancer patients who fulfill the Amsterdam criteria or met at least one of the revised Bethesda criteria. MSI and activating mutations of BRAF and KRAS were identified in all tumor samples. Methylation of the 5’ regulatory MLH1 region C at −197 to −295 bp (from the transcriptional start site) was analyzed by high-resolution melting (HRM) and also was performed methylation specific-MLPA (MS-MLPA) in all MSI-H cases.

Results: The average age of colorectal patients was 46 (29 to 79) years. MSI-H, BRAF and KRAS mutations were observed in 16, 1 and 14 out of all cases, respectively. Tumor localization was right-side in 9 of the MSI-H cases. Hypermethylation of the MLH1 promoter was found in 2 MSI-H tumors with KRAS mutation and right-side tumor localization.

Conclusion: The patients selection for the study is ongoing. Methylation analysis of MLH1 can give better performance characteristics in the selection of patients for sequencing of MLH1.